Continued development and purification of analogues of human calcitonin is planned with a view to (1) increase of stability as regards in vivo degradation and (2) increased potency. The completion of synthesis and purification of an intrinsically labelled derivative of human calcitonin with high specific activity is also anticipated. With the aid of these preparations the probe of the mechanism of (1) action of calcitonin as well as (2) the termination of action will be continued in in vitro defined systems of isolated target cells. Exploration will be conducted in man of more durable and effective derivatives of calitonin for control of metabolic bone diseases such as Paget's disease or osteoporosis.